Dont forget this year, the State of the Map (SOTM) is from the 9th to 11th of September 2011 in Denver, just before FOSS4G 2011. You got two weeks of FOSS and data for GIS.
–
ThomasG77Jan 7 '11 at 18:56

I've attended two ESRI UCs and got to go to the ESRI Dev Summit in Palm Springs this year. If you are drinking the ESRI Kool-Aid, then the Dev Summit is your best bet over the UC, in my opinion. Everything at the Dev Summit is developer-centric. Code everywhere. ESRI engineers everywhere. The engineers are at the UC as well, but not in such a massive concentration as at the Dev Summit. Plus, the Dev Summit is a little bit more intimate (like 3,000 people as opposed to 14,000). Not to mention Palm Springs in March is awesome.

State of The Map is a conference that gathers people using/mapping/interested in OpenStreetMap. It might be labeled as less technical, but nevertheless a lot of developers from all around the world attend this event every year. For example, this year lot of people from MapQuest engineering team were there.

I've gone to GeoWeb last two years. The presentations may not all be technical, but at least for these two years there have been some really smart people there. Worth attending just for hallway conversations. Unfortunately, this part depends a lot on who else shows up :)
–
JasonBirchJul 24 '10 at 10:25

If you're a member of ACMSIGSPATIAL, I found that you get regularly notified of a variety of upcoming conferences via call for papers. Not everything is strictly GIS related in the classical sense (indoor positioning technologies is a topic as well, for instance), but it is one option for keeping ajour with the research and conference scene.